1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, an LCD device with a buried touch screen.
2. Background of the Related Art
With the advance of portable electronic devices such as mobile communication terminals and notebook computers, flat panel display (FPD) devices have been in great demand because the LCDs used in the FPD devices are easily manufactured and have easy drivers, high image quality and large screens.
An LCD device includes a lower substrate and an upper substrate, and a liquid crystal layer disposed between the lower and upper substrates. The LCD device controls transmittance of light that transmits through a plurality of pixel cells in the liquid crystal layer by applying a data voltage, to thereby display an image based on a video signal.
Recently, touch screens have become popular as substitutes for input devices such as mouse or keyboards because they allow users to directly input information by fingers and/or pens. The touch screens are also suitable for monitors such as navigation, industrial terminals, notebook computers, financial automation equipment and game machines, portable terminals such as portable phones, MPEG Audio layer 3 (MP3) players, personal digital assistants (PDAs), portable multimedia players (PMPs), play station portables (PSP), portable game machines and digital multimedia broadcasting (DMB) receivers, and home appliances such as refrigerators, microwave ovens and washing machines. The application of the touch screens are being expanded because they are easy to operate.
Recently, LCD devices with the buried touch screens have been developed to be slim. FIGS. 1 and 2 are views schematically illustrating a related art LCD device with an interred touch screen.
The related art LCD device, as illustrated in FIGS. 1 and 2, includes a lower substrate 50 and an upper substrate 60 that are coupled with a liquid crystal layer (not shown) therebetween. The related art LCD device controls light that transmits through a plurality of pixel cells in the liquid crystal layer according to a data voltage, and displays an image based on an video signal. When a user uses a finger or a pen to touch a screen on the upper substrate 60, the related art LCD device detects a touch point with the change of a capacitance Ctc.
The upper substrate 60 includes a black matrix 62 that defines a pixel region so as to be in correspondence with each of a plurality of pixel cells, a red color filter 64R that is formed in a pixel cell defined by the black matrix 62, a green color filter 64G that is formed in a pixel cell defined by the black matrix 62, a blue color filter 64B that is formed in a pixel cell defined by the black matrix 62, and an overcoat layer 66 that is formed to cover the black matrix 62 and the color filters 64R, 64G and 64B and planarizes the upper substrate 60. The lower substrate 50 includes a plurality of pixel cells for detecting the touch of the user's finger or the pen, and drives the liquid crystal layer.
As shown in FIG. 2, each of the pixel cells is defined by a data line 42 and a gate line 44 that intersect each other. The pixel cell is formed with a common electrode 46 to receive a common voltage and a pixel electrode 48 to supply a data voltage to the pixel cell. Herein, the common electrode 46 and the pixel electrode 48 are formed of a transparent conductive material such as indium tin oxide (ITO). Also, in the pixel cell, a thin film transistor TFT is switched with a gate signal that is applied through the gate line 44, and an electric field is formed with a data voltage that is applied to the data line 42 to thereby drive the liquid crystal layer.
At this point, the LCD device drives the common electrode 46 as a sensing line/driving line to detect a touch point of a user's finger or a pen, for a non-display duration. That is, a touch capacitance Ctc is formed between the upper substrate 60 and the common electrode 46 of each pixel cell according to the touch point. A touch sensor of the LCD device compares the touch capacitance Ctc based on the touch point with a reference capacitance, thereby detecting a touch point TS and outputting the detected touch point TS to the outside.
As described above, in the related art LCD device with the interred touch screen that uses a common electrode as a touch electrode, the common electrode is divided into an n number of horizontal common electrodes and an m number of vertical common electrodes for recognizing a touch coordinate. In this case, the common electrodes are required to be connected to each other in display, but be disconnected from each other in driving for touch recognition. An operation of connecting and disconnecting the common electrodes is performed in a Vcom multiplexer unit of a printed circuit board (PCB).
In the related art LCD device with the interred touch screen, since the common electrodes are disposed with different sizes and types, the common electrodes have different electric potentials when the panel is not driven. The common electrodes having different electric potentials input different amounts of mobile charges to respective pixel cells. This causes block dim where a partial region of the liquid crystal panel is seen darker than another region thereof when the LCD device is initially driven. The block dim due to the mobile charges is moderated with the elapse of time, but as described above, the block dim is clearly shown when the LCD device is initially driven, thus causing the deterioration of image quality.